THE PLATE TECTONICS : THE DYNAMIC NATURE OF EARTH 🌎

 The Plate Tectonics : The            Dynamic Earth

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Image credit--USGS

Introduction

In Geologic terms, a plate is a large rigid slab of solid rocks. The word "Tectonics" comes from the Greek root to build. Putting these two words to together, we get the term Plate Tectonics, which refers to How the Earth's surface is built of Plates. In much the same way that geographic borders have separated, collided and been redrawn throughout human history, tectonics plate boundaries have diverged, converged and reshaped the Earth throughout it's geologic history. Today, Science has revealed that the Earth's surface is in a constant state of change. We are able to observe and measure mountains risings and eroding, ocean expanding and shrinking, volcanoes erupting and earthquakes striking. In order to explain these fatal natural disasters the whole Geo-scientific community started to search for the reason why such geologic events occur? Why Earth is so relentless?Why some regions on Earth are frequently struck by Earthquakes, Volcanoes, and many other Geologic events which are related to Plate tectonics? 

Why Earthquakes, Volcanoes and other geologic event occurs on our Planet?What ancient people thought about such geologic disasters?What were the ideas before Plate Tectonic?

The ancient people believed,these natural disasters occurs when God ( a belief in supreme power).In India ancient people believed that the DHARTI mata( The goddess of Land) would be very curious on human and his anger's reflectance came to be in the form of Earthquakes.In the same way ,AGNI mata( The Goddess of Fire) would be furious,then we see his anger as wild eruption of Volcanoes. There are a number of older ideas that people tossed around for many years.For example-- before 18th century many cultures envisioned that supernatural beings or supernatural processes were responsible for Earth's motion. In ancient Japanese culture it was thought that there was a Gigantic Catfish under Japan, who's wriggles and wiggles shook the ground and created Earthquake and uplifted mountains. But until the 20th century,Geologist threw around many ideas about Tectonic process none of which actually turned out to be correct.

Ideas before Continental Drift Hypothesis

Three of the most popular ideas that were held prior to our modern way of thinking ,was first of all that the Earth had mountain belts.Because since it's beginning,there have been long lived CRACKS OR WEAKNESS,in the crust .And at times molten rock pushed it's way up through these cracks and uplifted mountains belts at the surface.Another idea that was popular for a long period of time was the concept of CONTRACTION. People envisioned that the Earth was like a Baked Apple,just taken out of Oven.Which ,when it undergoes cooling shrinks. And those wringles were thought to be analogous to mountain belts. That's called Earth's Contraction hypothesis. Like such many theories developed none explained scientifically ,Why Earth is so Relentless?

And here comes a firm and plausible explanation by 32-year old German meteorologist Alfred Lothar Wagner.

THE CONTINENTAL DRIFT HYPOTHESIS

Image credit -USGS

Wagner had been a soldier in World War 1st,and after recovering from wounds he pondered over various geologic features and it consequences around the world,and he then came out with a Hypothesis of Continental Drift. The key feature of Wagner's hypothesis was the concept of moving continent which was totally absurd for Geologist of his contemporary time. Because more or less the whole Geo-Scientific community of his time of FIXIST view i.e. they believed that continents had been in constant state of rest,they are static. The concept of Moving Continent had been one of most revolutionising idea put forth by Wagner and it created a unrest in the whole Geo-Scientific community who at the time reluctant to to digest the concept of Moving Continent.

Alfred Wagner contended that, around 200 Ma ,the supercontinent Pangea( a Greek word meaning huge land mass) begin to split apart which was surrounded by huge watermass called PANTHALASSA. There was no trace of Ocean like Atlantic,Pacific,Indian and many other which are recent incident on  Geologic time scale.

Alexander Du Tour,Professor of Geology at Witwatersrand University and one of Wagner's staungest supporters proposed that Pangea first broke into two large Continental landmasses LAURASIA in the northern hemisphere and GONDWANALAND in the southern hemisphere which further broke apart into various smaller continent that exist today and India is a part of LAURASIA.

Wagner put forward numerous facts and findings to support his hypothesis and prove that it was rationale against his contemporaries. He first presented his hypothesis to the  German Geological Society on 6 January,1912 with following evidences-evidences-

1. The fit of Continental Shorelines

2. Distribution of late Paleozoic Glacial Sediments( 360-260 Ma)

3. Position of Paleo-climate belt

4. Distribution of Fossil species

5. Distribution of Rocks

1.The fit of Continental Shorelines---

Looking at this map, you can see that if you cut the continents, remove  the Atlantic Ocean, and fit them together, that the Northwest coast of Africa nuzzles tightly against the East coast of the United States. And that the swell of eastern South America fits well into the indentation on the west coast of Africa. There are remarkably fees overlaps and gaps in this construction. Wagner considered this fit to more than coincidence and decided that this could be used as proof that perhaps in the past that continents had fit together.

2.Distribution of late Paleozoic Glacial Sediments( 360-260 Ma) --As Wagner was meteorologist, he thought about climate, he thought about consequences of climate in the past. So, Wagner began to look for the evidence for where ice ages, meaning periods of time during which substantial areas of continental crust were covered by glaciers or sheets of ice, existed in the past. But Wagner realized from looking at the literature at that there were large examples or large areas that were covered by glacial sediments, meaning sediments or debris that were transported by ice sheets. In, fact Wagner realized, based on the information available at the time, that these deposits were of late Paleozoic age, meaning, they were over a couple hundred million years old. 

3.Position of Paleo-climate belt--

Wagner drew climate belt on his map of Pangea and then compared to what was know at the time about the distribution of rock types whose character was indicative of particular climate region. Some deposits of coal, sandstones made no sense on modern map of the world but they did make sense on Wagner's map of Pangea. So , we considered this to be also evidence in support of the model. 

4.Distribution of Fossil species--

Wagner had also did lots of work on fossil species,and Paleontologists, people who study fossils, realized that there were some fossils that were indicative of marine organisms. Now if we look at the world today , it's clear that there are certain kind of land organisms that only occur on certain continents and not on others. Because they can't swim, they can't get across the vast expanses of ocean that separate continents. For example-- there are kangaroos on Australia, but there are no kangaroos in North America even in comparable climates. For example--species of amphibians, species of plants, and other kinds of organisms, that could only have grown on land and looked at where they were distributed. Once again, they were occurring on all continents, really easy if you could walk from one continent to other, virtually impossible if you had to cross an ocean. 

5.Distribution of rocks--

Finally Wagner decided to look into distribution of distinctive packages of rock. Wagner was able to recognize distinctive assemblages of of rock that occurred either side of the Atlantic Ocean. And when he closed up the Atlantic Ocean and put Pangea back together, these packages of rock were immediately adjacent to each other as if they were once together and later split apart. 

All,the above evidences were plausible,but some of them were really unreasonable and the biggest flaw was lack of scientific mechanisms that how continent moves?The question on such topics by his critics could not be answered explicitly by Wagner and his hypothesis was discarded for many years but some of his findings were really remarkable.

Video credit -- IRIS

Arthur Holmes championed the theory of Continental Drift at a time when it was deeply unfashionable. He proposed in 1931 that the Earth's mantle contained Convection Cells which dissipated heat produced by Radioactive decay and moved the crust at the surface. Later discoveries added more evidence and made the Continental Drift theory rationale,the discovery of APPARENT POLAR WANDERING, and proof of SEA FLOOR spreading by Seafloor Bathymetry and finally converted the Continental Drift theory into more refined version in the form of Plate TECTONICS.

A very good animation

Most Geologist accept Plate Tectonic theory in part because the evidence for it is overwhelming and it ties together many seemingly unrelated Geologic features and events and showed how they are interrelated. Consequently, Geologist now view such Geologic processes as Mountain building, Earthquake,and Volcanism from the perspective of Plate Tectonics. Furthermore,because all the inner planets have had a similar origin and early history,Geologist are interested in  determining whether plate Tectonics is unique to Earth or whether it operates in the same way on the planets.